Controller for motor power steering system

ABSTRACT

In a control unit for an electric power steering apparatus, a mechanical system and a control system are designed to match a desirable complementary sensitivity function, and the steering is improved to obtain stable and comfortable steering feeling. For this purpose, there is provided a control unit for an electric power steering apparatus that controls a motor for giving steering auxiliary force to a steering mechanism based on a current control value calculated from a steering auxiliary command value calculated based on the steering torque generated in the steering shaft, and a current value of the motor. In this control unit, the complementary sensitivity function relative to a frequency is set to a level that approaches 1 in a band in which disturbance to be suppressed exists, and is set to a level that approaches 0 in a band in which disturbance to be transmitted exists.

TECHNICAL FIELD

[0001] The present invention relates to a control unit for an electricpower steering apparatus that provides steering auxiliary force by motorto the steering system of an automobile or a vehicle. The inventionparticularly relates to a control unit for an electric power steeringapparatus in which a desirable complementary sensitivity function isobtained, and a mechanical system and a control system are designedbased on the complementary sensitivity function.

BACKGROUND ART

[0002] An electric power steering apparatus that applies auxiliary loadto the steering apparatus of an automobile or a vehicle with turningeffort of a motor applies the driving force of the motor to a steeringshaft or a rack axis based on a transmission mechanism like gears orbelts via a reduction gear. Such a conventional electric power steeringapparatus carries out a feedback control of a motor current foraccurately generating an assist torque (a steering auxiliary torque).The feedback control is for adjusting a motor application voltage so asto minimize a difference between a current control value and a motorcurrent detection value. The motor application voltage is generallyadjusted based on a duty ratio of a PWM (Pulse Width Modulation)control.

[0003] Approximately ten years have passed since the electric powersteering apparatus appeared in the market, and the application of thisapparatus has now been expanded to a vehicle class of 2,000 cc. At thesame time, high-level performance of the steering apparatus has alsobeen required. Recently, not only the performance of the electric powersteering apparatus has reached the performance level of the conventionalhydraulic power steering apparatus, new functions of the electric powersteering apparatus have been developed, aiming at the advent of newvalue-added products.

[0004] A general structure of the electric power steering apparatus willbe explained below with reference to FIG. 28. A shaft of a steeringwheel is connected to a tie rod of running wheels through a torsion bar,a reduction gear, universal joints, etc. The shaft of the steering wheelis provided with a torque sensor for detecting a steering torque of thesteering wheel. A motor for assisting the steering force of the steeringwheel is connected to the shaft through a clutch (not shown) and thereduction gear. A control unit (ECU) for controlling the power steeringapparatus is supplied with power from batteries through an ignition key(not shown). The control unit calculates a steering auxiliary commandvalue I of an assist command based on a steering torque T detected bythe torque sensor and a vehicle speed V detected by a vehicle speedsensor (not shown). The control unit then controls a current to besupplied to the motor based on the calculated steering auxiliary commandvalue I . In FIG. 28, SAT represents a self-aligning torque.

[0005] The control unit is mainly composed of a CPU. FIG. 29 showsgeneral functions to be executed based on a program inside the CPU.

[0006] Functions and operation of the control unit 30 will be explainedbelow. A steering torque T detected by the torque sensor 10 and theninput is phase-compensated by the phase compensator 31 for increasingthe stability of the steering system. The phase-compensated steeringtorque TA is input to a steering auxiliary command value calculator 32.A vehicle speed V detected by the vehicle speed sensor 12 is also inputto the steering auxiliary command value calculator 32. The steeringauxiliary command value calculator 32 calculates a steering auxiliarycommand value I as a control target value of a current to be supplied tothe motor 20, based on the input steering torque TA and the inputvehicle speed V. The steering auxiliary command value I is input to asubtracter 30A, and is also input to a differential compensator 34 of afeedforward system for increasing a response speed. A difference (I−i)calculated by the subtracter 30A is input to a proportional calculator35, and is also input to an integration calculator 36 for improving thecharacteristic of a feedback system. Outputs from the differentialcompensator 34 and the integration calculator 36 are input to and addedtogether by an adder 30B. A result of the addition by the adder 30B isobtained as a current control value E, and this is input to a motordriving circuit 37 as a motor driving signal. A motor current value i ofthe motor 20 is detected by a motor current detecting circuit 38, andthis motor current value i is input to the subtracter 30A and is fedback.

[0007]FIG. 30 is a block diagram showing the transmission function ofthe electric power steering apparatus. In the drawing, “s” represents aLaplace operator.

[0008] Particularly, the electric power steering apparatus has anadvantage that it is possible to process information from the roadsurface and transfer the information to the driver to facilitate thedriving, by utilizing the degree of freedom of designing theinformation. From FIG. 28 and FIG. 30, it can be understood that thesensitivity designing of the road information can be handled as asubject of defining a desirable transmission characteristic from theinput of the road surface information to the delivery of the informationto the steering torque.

[0009] In the mean time, the steering feeling desirable for the driveris realized by tuning the transmission characteristic from the steeringangle to the steering torque. In general, the demand for clear steeringwheel or quiet steering feeling depends greatly on this transmissioncharacteristic. It can be understood from the schematic block diagramshown in FIG. 30 that these two specifications are in the trade offrelationship. For example, this corresponds to a case where the driverfeels friction in the steering as a result of the addition of frictionto the power steering system in order to lower the sensitivity to thewheel flutter that occurs during a high-speed running of the vehicle.

[0010] In the field of the electric power steering apparatus, there hasbeen demanded an advent of a product of which performance exceeds thatof the hydraulic power steering apparatus while satisfying theperformance of the electric power steering apparatus. Further, thisproduct is desired to obtain stable and comfortable steering wheel,based on the designing of a control system and an electric controlsystem that match the desirable complementary sensitivity function basedon the road information.

[0011] The present invention has been made in the light of the abovesituations. It is an object of the present invention to provide acontrol unit for an electric power steering apparatus in which adesirable complementary sensitivity function is obtained, and a controlsystem is designed to match this complementary sensitivity function.

DISCLOSURE OF THE INVENTION

[0012] The present invention provides a control unit for an electricpower steering apparatus that controls a motor for giving steeringauxiliary force to a steering mechanism based on a current control valuecalculated from a steering auxiliary command value calculated based onthe steering torque generated in the steering shaft, and a current valueof the motor. The object of the present invention can be achieved basedon the arrangement that the complementary sensitivity function relativeto a frequency is set to a level that approaches 1 in a band in whichdisturbance to be suppressed exists, and is set to a level thatapproaches 0 in a band in which disturbance to be transmitted exists.

[0013] Further, the object of the present invention can be achieved moreeffectively by the following arrangement. An eigenvalue of the powersteering apparatus, an eigenvalue of suspension, and a flutteroscillation area and a motor torque ripple area are included in the bandin which the disturbance to be suppressed exists. Alternatively, theeigenvalue of the power steering apparatus is set to 10 to 13 Hz, theeigenvalue of suspension is set to 13 to 17 Hz, the flutter oscillationarea is set to 15 to 25 Hz, and the motor torque ripple area is set to15 to 30 Hz. Alternatively, the complementary sensitivity function isobtained from a design of a mechanical control system and an electriccontrol system. Alternatively, the mechanical control system is obtainedfrom designs of a rolling-type rack and pinion mechanism, a rubberdamper of a motor reduction gear mechanism, and a non-contact torquesensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a characteristic diagram showing a desirablecomplementary sensitivity function; FIG. 2 is a simplified blockstructure diagram of FIG. 30; FIG. 3 is a frequency response diagramshowing a motor characteristic; FIG. 4 is a diagram showing an exampleof a tuning of a transmission characteristic from a road surface inputto a steering torque; FIGS. 5 (A) and (B) are diagrams showing examplesof a measuring of a steering torque (a hydraulic system) when a vehiclehas run on a Belgian road; FIGS. 6 (A) and (B) are diagrams showingexamples of a measuring of a steering torque (an electric system) when avehicle has run on a Belgian road; FIGS. 7 (A) and (B) are diagramsshowing examples of a steering characteristic when a vehicle has run at100 Km/h; FIG. 8 is a mechanism diagram of a rolling-type rack andpinion that is used in the present invention; FIG. 9 is a diagramshowing an example of a characteristic of the mechanism of FIG. 7 incomparison with that of a conventional apparatus; FIG. 10 is a mechanismdiagram of a conventional rolling-type rack and pinion; FIG. 11 is amechanism diagram of a worm shaft supporting section of a motorreduction gear; FIG. 12 is a diagram showing a frequency characteristicwhen a rubber is not mounted on a worm shaft supporting section; FIG. 13is a diagram showing a frequency characteristic when a rubber is mountedon a worm shaft supporting section; FIG. 14 is a cross-sectionalmechanism diagram of a torque sensor that is used in the presentinvention; FIG. 15 is a perspective view of a torque sensor that is usedin the present invention; FIG. 16 is a block diagram showing an exampleof a structure of an electric power steering apparatus according to thepresent invention; FIG. 17 is a block structure diagram of a centerresponse improving section; FIG. 18 is a diagram showing an example of acharacteristic of a phase compensating section; FIG. 19 is a diagramshowing an example of a characteristic of an approximate differentiatingsection; FIG. 20 is a diagram showing a combined characteristic of thephase compensating section and the approximate differentiating section;FIG. 21 is a diagram showing an example of a vehicle speed and a settingof gain based on a steering torque; FIG. 22 is a diagram showing a basicassist characteristic; FIG. 23 is a diagram showing an example of avehicle speed interpolated calculation; FIG. 24 is a block diagramshowing an example of a structure of a torque control calculation; FIG.25 is a diagram showing an example of a characteristic of robuststabilization compensation; FIG. 26 is a diagram showing an example of acharacteristic of a control system; FIG. 27 is a diagram showing anexample of a characteristic of a mechanical system; FIG. 28 is amechanism diagram showing a general example of an electric powersteering; FIG. 29 is a block diagram showing a general internalstructure of a control unit; and FIG. 30 is a block diagram showing atransmission function of the control unit.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] According to an electric power steering, there exists an electricmotor that does not exist in a hydraulic power steering, in the middleof transmitting road information to the driver. The friction and inertiaof the motor block almost all road surface information. Thus, theelectric power steering has a structure that does not easily transmitthe road information to the driver. On the other hand, the electricpower steering has an advantage that the existence of the motorinterrupts unnecessary disturbance. The present inventor has beenengaged in the study of an electric power steering apparatus for manyyears, and has found that the road information can be broadly dividedinto necessary information and unnecessary information that should besuppressed as disturbance, as shown in FIG. 1. Namely, a complementarysensitivity function T (s) in relation to the frequency that the driversenses is always equal to or lower than 1. The complementary sensitivityfunction T (s) has a flat linear characteristic close to 0 dB=1, in thearea in which there exist an eigen frequency (10 Hz to 13 Hz) of theelectric power steering apparatus, and an eigen frequency (13 Hz to 17Hz) of a suspension, and in a wheel flutter oscillation area (15 Hz to25 Hz) and a motor torque ripple area (15 Hz to 30 Hz). In other areas,the complementary sensitivity function T (s) has an inclinedcharacteristic coming close to zero.

[0016] The electric power steering apparatus can be designed on afrequency area, and therefore, it is possible to solve the trade off bydistinguishing it on the frequency area of the complementary sensitivityfunction in FIG. 30. In other words, according to a satisfactorysteering, it is possible to suppress unnecessary disturbance, andtransmit necessary disturbance to the steering wheel. According to theconventional hydraulic power steering, for example, this point is copedwith by adjusting the friction in the steering system. However, it isnot possible to satisfy both at the same time. On the other hand,according to the electric power steering, it is possible to define thetransmission characteristic from the road surface to the steering wheel.Therefore, it is possible to solve the trade off in the frequency area.Specifically, the complementary sensitivity function T (s) of thecontrol system is set close to “1” in the area in which there existsdisturbance that should be suppressed. The complementary sensitivityfunction T (s) is set close to zero in the area in which there existsdisturbance that should be transmitted. In other words, from thedefinition of the complementary sensitivity function T (s), disturbanceis suppressed completely when the complementary sensitivity function T(s) is “1”, and disturbance is transmitted without being suppressed atall when complementary sensitivity function T (s) is zero.

[0017] The complementary sensitivity function is calculated based on theassumption that a vehicle has a simple spring (a spring multiplier Kv)as a transmission characteristic for the vehicle to generate a SAT(self-aligning torque). A constant gain becomes K/Kv/α² . In the bandwhere there exists disturbance to be suppressed, the complementarysensitivity function becomes close to 1. In defining the complementarysensitivity function, FIG. 30 is interpreted as follows. FIG. 30 can beinterpreted to show a control system that carries out a control toreduce the displacement of the torsion bar. Reducing the displacement ofthe torsion bar is equivalent to reducing the steering torque from FIG.30. Therefore, the control system can be regarded as the one that feedsback θ_(b) with θ_(h) as a target, and has a control gain K (rigidity ofthe torsion bar) and the electric power steering. The constant gain ofthe controller of the electric power steering becomes a gradient of theassist characteristic. Therefore, the constant gain is zero in the areawhere the torque is small. When a new controller C (s) is provided bycombining the rigidity of the torsion bar and the controller, and asteering system is expressed as P (s), the control system can besimplified as a general control system as shown in FIG. 2. Thecomplementary sensitivity function T (s) is expressed by the equationshown in FIG. 2, where d(s) represents the disturbance coming from thetires. This disturbance d (s) includes unnecessary disturbance and adifference between the characteristic of the vehicle and the dynamiccharacteristics of the spring. Therefore, the object of thecomplementary sensitivity function T (s) is to transmit the differencebetween the dynamic characteristics of the simple spring and the actualdynamic characteristics in a suitable band, and to suppress theunnecessary disturbance at the same time.

[0018] In the present invention, attention has been paid to thedifference between the transmission characteristic from the road surfaceinformation to the steering torque and the transmission characteristicfrom the steering angle to the steering torque. The inertia of the motoris positively utilized for unnecessary disturbance, and the inertia ofthe motor sensed at the time of the steering is compensated for in thetorque control system. In a gain diagram shown in FIG. 3, transmissioncharacteristics from the steering angle to the steering torque arecompared based on the inertia of the motor (high inertia, and lowinertia). From a motor frequency response shown in FIG. 3, it is clearthat the influence of the motor inertia appears as a phase delaycharacteristic. The influence of the motor inertia can be compensatedfor in the torque control system, by using a phase advancecharacteristic that becomes an inverse characteristic of the phase delaycharacteristic. In the phase diagram shown in FIG. 3, a characteristic Ashows a case where there is no compensation, and a characteristic Bshows a case where there is compensation.

[0019] In designing the road surface sensitivity, it is necessary thatthe torque sensor can detect the road surface information. Therefore,the structure is arranged such that the motor is prevented frominterrupting the road surface information. Then, the torque controlsystem is designed such that its complementary sensitivity functioncomes close to the characteristic shown in FIG. 1. The road surfaceinformation generated on the tires is transmitted after the frictioncomponent at each element is subtracted. As the motor inertia works as amechanical low-pass filter, the road surface information is attenuatedwhen the inertia is large. The designing of the complementarysensitivity function is fine-tuned to match the vehicle aftersufficiently securing the stability of the control system by applying ageneral control system designing method. This is because the humansensitivity is subtle, and it is not possible to express idealcharacteristic in the transmission characteristic. According to ageneral designing method that tends to be conservative, it is notpossible to take sufficient measure. Consequently, under the currentsituation, the tuning itself depends on the know-how of the tuningtechnician.

[0020] On the other hand, according to the present invention, in orderto realize the complementary sensitivity function shown in FIG. 1, therolling-type rack and pinion, the rubber damper of the motor reductiongear, the non-contact torque sensor, and the torque control system areimproved. These will be explained sequentially below.

[0021] In order to prevent the motor from interrupting the road surfaceinformation, the following elements (1) to (3) are effective. (1)Defining a motor characteristic suitable for road surface information byfeeding back a status using an observer. (2) De-coupling the motor fromthe column shaft by employing a mechanical clutch mechanism. (3)Employing a low-friction element. FIG. 4 to FIG. 7 show examples ofcharacteristics of an electric power steering apparatus designed on thisbasis, in comparison with the characteristics of the hydraulic system.FIG. 4 shows an example of a measurement of tuned road surfacesensitivity. A thick line shows sensitivity (dB), and a thin line showsphase (degree). FIG. 5 and FIG. 6 are diagrams showing examples of ameasuring of a steering torque of a hydraulic system and a steeringtorque of an electric system when a vehicle has run on a Belgian roadrespectively. The reason why the steering torque of the hydraulic powersteering apparatus has varied is that the oscillation of the suspensionis detected. FIG. 5 (A) and FIG. 6 (A) show variations in the steeringtorque in relating to time, and FIG. 5 (B) and FIG. 6 (B) show frequencyresponses in 0 to 60 Hz. FIG. 7 (A) shows an example of a measurement ofa steering characteristic (steering torque in relation to steeringangle) of the hydraulic power steering apparatus when a vehicle has runat 100 Km/h. FIG. 7 (B) shows an example of a measurement of a steeringcharacteristic (steering torque in relation to steering angle) of theelectric power steering apparatus when a vehicle has run at 100 Km/h.The reason why the steering torque of the hydraulic power steeringapparatus has varied is that the oscillation of the flutter is detected.

[0022]FIG. 8 shows a mechanism of a rolling-type rack and pinion that isused in the present invention. FIG. 9 shows a comparison between acharacteristic of the mechanism in comparison with that of aconventional mechanism. A pinion shaft is coaxially engaged with aninput shaft. The pinion shaft is engaged with a rack shaft. The rackshaft is connected with a pin shaft of a pressure pad within a housingvia a roller. The pin shaft is held by needle bearings, and is connectedwith a coil spring via a friction block. The coil spring is accommodatedin a holder, and the pressing force is applied to the pressure pad. Theholder is elastically held by a spring suspended between inner walls ofthe housing. This rolling-type rack and pinion is described in detail inJapanese Patent Application Laid-Open No. 2000-159128 by the presentapplicant.

[0023] According to the rolling-type rack and pinion of the presentinvention, the pressure pad for supporting the pinion consists of theroller, the friction block, the needle bearings, and the holder.Therefore, it is possible to meet both high supporting rigidity and lowwork resistance. Particularly, as the friction block is disposed in thepressure pad, a reverse input in the area of low rack force is lowerthan the reverse input according to the conventional rack and pinionmechanism shown in FIG. 10, as is clear from the characteristic diagramshown in FIG. 9. The provision of this friction block is useful forimproving the road surface information in the minute steering angle areathat is important for the high-speed running. According to theconventional mechanism (having no friction block) shown in FIG. 10, asthe friction block is not disposed in the pressure pad, the reverseinput becomes high in the area of low rack force.

[0024] The rubber damper in the motor reduction gear according to thepresent invention will be explained next.

[0025] As shown in FIG. 11, according to the electric power steeringapparatus, a rubber damper (rubber) is inserted into a spline via a bushin the worm shaft supporting section of the motor reduction gear, inorder to reduce rattle noise of the gear. In the mean time, in therubber elastic area, the displacement of the motor and the displacementof the column shaft work independent of each other. Therefore, it ispossible to transmit the road surface information to the steering wheelshaft without being interrupted by the friction and the inertia of themotor. As a result, based on this mechanism, it is possible to realizethe design of the sensitivity function of the road surface informationas shown in FIG. 1. However, the insertion of the rubber damper leads toa high-dimensional structure of the controller, as the control item hasdynamic characteristics of a low eigen frequency.

[0026]FIG. 12 shows a frequency characteristic when a rubber damper isnot mounted on a worm shaft supporting section. FIG. 13 shows afrequency characteristic when a rubber damper is mounted on a worm shaftsupporting section. It can be understood that the noise level is reducedin the latter case.

[0027] The improvement of the torque sensor that is used in the electricpower steering will be explained next.

[0028] As the hysteresis characteristic of the detection characteristicof the torque sensor appears as a delay characteristic in the finetorque, it is necessary to suppress this hysteresis characteristic assmall as possible. For this purpose, according to the present invention,a non-contact torque sensor having a small hysteresis width will be usedas shown in FIG. 14 and FIG. 15. FIG. 14 shows a structure that anon-contact torque sensor is disposed in a steering wheel shaft FIG. 15is a perspective view of a partial cross section of the structure of thesensor. A bobbin yoke forming a detection circuit unit is disposed on asleeve at the external periphery of an input shaft (a sensor shaft)consisting of a magnetic material like SUS and Fe. Two sets of coils arewound within the bobbin yoke. The sleeve consists of a conductivenonmagnetic material (aluminum, for example). Windows are formed along aring-shaped coil string. A torsion bar is disposed inside the inputshaft.

[0029] Based on this structure, the torque to the input shaft isdetected in non-contact, by utilizing the conductivity and non-magnetismof the sleeve and the magnetism of the input shaft. In other words, aclose status of a periodical magnetic field is generated in theperipheral direction inside the sleeve by utilizing the skin effect.Spontaneous magnetization of the input shaft is increased or decreasedbased on a phase difference between the phase of the magnetic field andthe phase of the spline of the input shaft. A change in the impedancegenerated based on this operation is detected as a change in the voltageat the coil end by a bridge circuit formed by coils or the like.

[0030] The design of the electric control system will be explained next.

[0031] For realizing the sensitivity design of the road surfaceinformation, the responsiveness of the current control is also animportant element. Particularly, it is desirable that the responsivenessaround the time of staring a current flow is set as linear as possiblefrom the viewpoint of improving the steering near the neutral point. Forlinearizing the responsiveness, a robust control is employed based on astandard model, in place of the current control based on a conventionalPI controller.

[0032]FIG. 16 is a total block diagram showing control functions of thepresent invention. A steering torque T is input to a steering auxiliarycommand value calculating section 100 and a center responsivenessimproving section 101. Outputs from these sections are input to an adder102. A result of an addition by the adder 102 is input to a torquecontrol calculating section 103. An output signal from the torquecontrol calculating section 103 is input to a motor loss currentcompensating section 104. An output signal of the motor loss currentcompensating section 104 is input to a maximum current limiting section106 via an adder 105. A maximum current value limited by the maximumcurrent limiting section 106 is input to a current control section 110.An output of the current control section 110 is input to a currentdriving circuit 112 via an H bridge characteristic compensating section111. Based on this, the current driving circuit 112 drives a motor 113.

[0033] A motor current i of the motor 113 is input to a motor angularvelocity estimating section 121, a current drive switching section 122,and the current control section 110, via a motor current offsetcorrecting section 120. A motor terminal voltage Vm is input to themotor angular velocity estimating section 121. An angular velocity ωestimated by the motor angular velocity estimating section 121 is inputto a motor angular velocity estimating section/inertia compensatingsection 123, a motor loss torque compensating section 124, and a yawrate estimating section 125. An output of the yaw rate estimatingsection 125 is input to an astringency control section 126. Outputs ofthe astringency control section 126 and the motor loss torquecompensating section 124 are added by the adder 127. A result of theaddition is input to the adder 102. Further, a current dither signalgenerating section 130 is provided. Outputs of the current dither signalgenerating section 130 and the motor angular velocity estimatingsection/inertia compensating section 123 are added by an adder 131. Aresult of this addition is input to the adder 105.

[0034] Based on the above structure, according to the present invention,the center responsiveness improving section 101 consists of a phase tcompensating section 101A, an approximate differentiating section 101B,and a gain setting section 101C, as shown in FIG. 17. Further, the phasecompensating section 101A has a frequency characteristic as shown inFIG. 18, and the approximate differentiating section 101B has afrequency characteristic as shown in FIG. 19. With this arrangement, acombined characteristic of the phase compensation and the approximatedifferentiation becomes as shown in FIG. 20. The gain setting section101C sets the gain by switching the vehicle speed V and the steeringtorque T as shown in FIG. 21. Further, in order to reduce the unstablesteering feeling that the steering wheel is suddenly returned, and tostabilize the steering, the steering torque is increased, the steeringtorque change rate is increased, and the gain is decreased when thesteering torque is in the decreasing direction. In other words, theswitching condition is set as follows. |steering torque|(=A)>about 1.37Nm, |steering torque−steering torque (one sampling before)|(=B)>about0.137 Nm, and sign (A)<>sign (B). The gain after the switching is “46”at the vehicle speed 0 to 2, “47” at the vehicle speed 4 to 78, and “41”at the vehicle speed 80 or above, for example. In the above, sign(A)<>sign (B) means that the signs of (A=steering torque) and(B=steering torque−steering torque (one sampling before)) are different.

[0035] Further, according to the present invention, the steeringauxiliary command value calculating section 100 sets the assistcharacteristic of three representative vehicle speeds (0, 30, 254 Km/h)as a basic characteristic in the calculation of the assist value. Thesteering auxiliary command value calculating section 100 calculates theassist values at other speeds by interpolating between the basiccharacteristics for every 2 Km/h of the vehicle speed according to thevehicle interpolation gain. Then, the vehicle speed of the assistcharacteristic is set to a range from 0 to 254 Km/h, and the resolutionis set as 2 Km/h. FIG. 22 shows the basic assist characteristic (torqueversus current). The basic assist characteristic is expressed as 0Km/h=1o characteristic, 30 Km/h=1a characteristic, and 254 Km/h=1bcharacteristic. For other vehicle speeds, the assist current iscalculated by interpolating between the vehicle speeds for every 2 Km/husing a vehicle (Km/h) versus vehicle speed interpolation coefficient γshown in FIG. 23. When the vehicle speed is from 0 to 30 Km/h, theassist current I is I=1a(T)+γ(V)(1o(T)−1a (T)). When the vehicle speedis from 32 to 254 Km/h, the assist current I isI=1b(T)+γ(V)(1a(T)−1b(T)).

[0036] Further, according to the present invention, the torque controlcalculating section 103 sets a steering torque response for stabilizingthe mechanical system of the electric power steering apparatus,stabilizing the oscillation of the rubber damper at the reduction gear,and adjusting the steering feeling. FIG. 24 shows this structure. Aresponsiveness defining section 103B is provided at the rear stage of aclamp circuit 103A. At the rear stage of the responsiveness definingsection 103B, a robust stabilization compensating section 103D isdisposed via a clamp circuit 103C. At the rear stage of the robuststabilization compensating section 103D, a phase compensating section103F is provided via a clamp circuit 103E. Further, a robuststabilization compensating section 103H is disposed via a clamp circuit103G.

[0037]FIG. 25 shows a characteristic of the robust stabilizationcompensating section 103H, and FIG. 26 shows a total characteristic ofthe control system. FIG. 27 shows a characteristic of the mechanicalsystem. In total, the crest and trough are cancelled, and asubstantially flat characteristic is obtained.

[0038] Further, according to the present invention, in order to improvethe start-up from the motor output torque 0 by adding, a current thatdoes not appear as a motor output even when the motor current flows isset, as a tuning of the center feeling. For this purpose, thecompensation value is added to have the same sign as the sign of thetorque control calculation output. The compensation value is switched atfour stages based on the vehicle speed.

[0039] Industrial Applicability

[0040] As described above, according to the present invention, adesirable complementary sensitivity function is obtained from the roadsurface information, and the mechanical system and the current controlsystem are designed based on the complementary sensitivity function.Therefore, it is possible to prevent an unnatural steering feeling, andobtain a comfortable steering feeling.

1. A control unit for an electric power steering apparatus that controlsa motor for giving steering auxiliary force to a steering mechanismbased on a current control value calculated from a steering auxiliarycommand value calculated by a calculating unit based on the steeringtorque generated in the steering shaft, and a current value of themotor, wherein the complementary sensitivity function relative to afrequency is set to a level that approaches 1 in a band in whichdisturbance to be suppressed exists, and is set to a level thatapproaches 0 in a band in which disturbance to be transmitted exists. 2.The control unit for an electric power steering apparatus described inclaim 1, wherein an eigenvalue of the power steering apparatus, aneigenvalue of suspension, and a flutter oscillation area and a motortorque ripple area are included in the band in which the disturbance tobe suppressed exists.
 3. The control unit for an electric power steeringapparatus described in claim 2, wherein the eigenvalue of the powersteering apparatus is set to 10 to 13 Hz, the eigenvalue of suspensionis set to 13 to 17 Hz, the flutter oscillation area is set to 15 to 25Hz, and the motor torque ripple area is set to 15 to 30 Hz.
 4. Thecontrol unit for an electric power steering apparatus described in anyone of claims 1 to 3, wherein the complementary sensitivity function isobtained from a design of a mechanical control system and an electriccontrol system.
 5. The control unit for an electric power steeringapparatus described in claim 4, wherein the mechanical control system isobtained from designs of a rolling-type rack and pinion mechanism, arubber damper of a motor reduction gear mechanism, and a non-contacttorque sensor.